At some point, every Linux release becomes EOL, that is, it reaches its end of life.
Becoming EOL means it will no longer be supported by the Linux community with security patches, vulnerabilities, or bug fixes.
CentOS 6 EOL Completed on November 30th, 2020. If you have servers running on CentOS 6 systems, make sure to update the servers to latest CentOS 7 x.x 64bit Operating System.
Therefore, continuing to use CentOS 6 after November could pose a significant security risk. That is why it is advisable never to use EOL operating systems.
In comparison, CentOS 7 will be supported until June 2024. Surprisingly, the CentOS 8 life-cycle has been cut short and its EOL has been scheduled for December 2021.